SPOILER ALERT: the following article contains massive spoilers, including the ending. If you have not yet seen the movie, proceed at your own risk, or better, come back to this article later!
Former couple Sara and Jackson board a single-engine plane for a routine and casual flight to their friend’s tropical island wedding. However, within minutes after takeoff, their pilot suffers a fatal heart attack, leaving them with no idea where they are, and no clue how to land the plane. With nothing but miles of ocean and sky in every direction, and a terrifying storm that’s about to envelop them, Sara and Jackson have only one shot — and there’s no going back.
SPOILERS AHEAD
Horizon Line Plot Summary and Synopsis
Sara is at an island bar and is getting ready to head home. She is with a boyfriend and he asks her for one more drink before she goes.
He orders, then heads out of the bar (presumably to the bathroom), and she leaves, as she has stated that she “hates goodbyes.”
Fast forward one year later in London, Sara takes a cab to the airport to leave for a wedding.
Visiting Mauritius for her friend’s wedding, Sara hooks up with ex-boyfriend Jackson, misses the last boat to the wedding venue on Rodrigues, and hitches a ride on a small plane piloted by her friend, Wyman, who is also going to the wedding. She is surprised when it turns out that Jackson has also arranged to fly with Wyman to the wedding.
Wyman invites Sara to the co-pilot seat while Jackson sleeps. During the flight Wyman has a heart attack and passes away, sending the plane into freefall.
Sara and Jackson recover, but the autopilot is damaged, and they are faced with the fact that Sara, who Wyman has taught some basics, will need to fly the plane to get them to safety.
They manage to contact Samuel, a local air traffic controller, on the shortwave radio, and he attempts to guide them to safety, but they have to fly through a storm first.
They manage to get through the storm but not before they damage their compass.
The plane is also starting to lose fuel at an alarming rate.
Jackson volunteers to climb out of the plane to try to patch a fuel line. While successful, he badly breaks his arm. The pair decides to shed weight, including the lifeless body of Wyman, to save fuel. Sara realizes they have several bottles of rum that they can use as extra fuel, and she attempts to manually refuel the plane by climbing out the window.
Sara manages the temporary refueling and climbs back into the plane. They try to raise Samuel, but the radio is silent. As the plane runs out of fuel, they spot a small island and attempt to glide the stricken plane to the island. The plane crashes into the sea and flips.
Sara rescues Jackson, who got stuck at the wing of the submerged plane, and both barely escape and swim to the barren island.
They make it to the island only to discover that it is just a sandbar that will shortly disappear as the tide rises.
As the sand bar disappears, they stay afloat, but Jackson, sensing he is sure to die, offers Sara his life vest. Sara declines, telling him they are a team.
Horizon Line Ending
Jackson professes his love for Sara as they sense the end is near, but just then a fishing vessel that heard their radio transmission arrives to rescue them.
Source: Wikipedia (CC BY-SA 3.0).